The measurement of intracranial pressure (ICP) is important in diagnosing and treating various pathophysiological conditions caused by head trauma, hemorrhage, tumors, inflammatory diseases and the like. Several techniques have been used to measure ICP. Conventional invasive ICP measurement techniques require surgical passage through the skull bone into the brain ventricles, parenchyma, or the region between the skull and dura matter to implant a measuring transducer.
A non-invasive ICP measurement technique has been suggested that determines displacements of the tympanic membrane of the ear. However, it has not been possible to obtain a good correlation with ICP because determination of ICP by this method is complicated by the compressible air space between the pressure source and the interrogation point.
Another non-invasive ICP measurement method measures the electromagnetic impedance response of the brain to induced fields, and correlates the response to ICP. Such techniques are disclosed in U.S. Pat. Nos. 4,690,149 and 4,819,648 to Ko.
Another non-invasive ICP measurement technique that has been attempted involves ultrasonic imaging to detect relative displacements of tissue boundaries within the brain. The displacements may be associated with fluid build-up and compression or dilation of brain vessels, which permits determination of ICP through an independent calibration of compressibility. An alternate non-invasive ultrasonic technique involves the measurement of blood flow in the carotid artery by ultrasonic excitation of the artery and determination of Doppler frequency shift.
Various types of ultrasonic ICP measurement techniques are disclosed in France Patent FR, A, 2318420 to Guiset, U.S. Pat. No. 3,872,858 to Hudson et al., U.S. Pat. No. 4,043,321 to Soldner et al., U.S. Pat. No. 4,971,061 to Kageyama et al., U.S. Pat. No. 4,984,567 to Kageyama et al., U.S. Pat. No. 5,388,583 to Ragauskas et al., U.S. Pat. No. 5,411,028 to Bonnefous, U.S. Pat. No. 5,617,873 to Yost et al. and U.S. Pat. No. 5,919,144 to Bridger et al. Such techniques involve the transmission of ultrasonic waves typically having frequencies on the order 0.1 MHz . . . 0.5 MHz or 1.0 MHz . . . 5.0 MHz into the intracranial media.
Each of the patents cited above is incorporated herein by reference.
Despite the above-noted attempts to develop non-invasive ICP measurement technique, a need still exists for a non-invasive ICP measurement apparatus and method which can accurately measure ICP absolute value and all possible ICP waves without skull penetration and which poses no health risks during long-term monitoring. On the other hand, ICP changes are caused by intracranial media components' volume changes. These components are arterial and venous blood, cerebrospinal fluid (CSF), brain tissues and intersticial fluid. For the targeted therapy of raised ICP, it is necessary to know which intracranial component has increased in volume. However, using known ultrasonic ICP measuring methods and apparatus, it is still impossible to identify which intracranial component is the cause of ICP increment. Therefore, a need exists for a method and apparatus for simultaneous measurement and monitoring intracranial blood volume, CSF volume, and brain parenchyma tissue volume. The only known method and apparatus for the measurement of blood volume inside the brain parenchymal acoustic path know to the inventors is U.S. Patent No. 5,388,583.
However, known ultrasonic non-invasive ICP measuring apparatus and methods have the many limitations. For example, the known methods and apparatus:                are affected by the head external tissues blood flow phenomena;        are uncomfortable due to direct contact of the rigid surface of the ultrasonic transducers with the patient's extracranial tissues;        can eliminate extracranial tissue volume pulsation with only limited accuracy due to a limited signal-to-noise ratio in the ultrasonic echo from the surface of skull measurement channels;        can require the device operator to manually perform interactive ultrasonic signal adjustment procedures that are too lengthy and too sophisticated for emergency room or intensive care unit situations; and        make it impossible to define the intracraniospinal compliance changes using known ultrasonic non-invasive methods and devices.        